1. Field of the Invention
This invention relates to a developing method for development of electrostatic images in electrophotography, electrostatic recording and electrostatic printing, particularly electrophotography, and a positively chargeable developer therefor.
2. Description of the Prior Art
Developing methods in electrophotography, electrostatic recording, etc. of the prior art may be classified broadly into the dry developing method and the wet developing method. The former is further classified into the method using two-component system developer and the method using one-component system developer. Those belonding to the two-component system developing method include various methods with different carriers for transporting toner, such as the magnet brush method using iron powder carrier, the cascade method using beads carrier and the fur brush method using fur.
On the other hand, those belonging to the one-component system method include the powder cloud method in which toner particles are used in atomized state; the contact developing method (also called as toner development) in which development is effected by contacting the toner particles directly with the electrostatic latent image surface; the jumping developing method in which toner particles are not contacted directly with the electrostatic latent image surface but toner particles are charged and permitted to jump due to the electrical field given by the electrostatic image toward the latent image surface; and the magnedry method in which electroconductive toner is contacted with the electrostatic latent image.
According to the two-component system developing method, a mixed developer of carrier particles and toner particles is necessarily used, and toner particles are generally consumed in amounts by far larger than the carrier particles in the course of developing procedure. As the result, the mixing ratio of both particles will be changed to cause changes in density of sensible images. Also, through deterioration of carrier particles which are hardly consumed after usage for a long time, image quality will be lowered. These are inherent drawbacks of the two-component system developing method.
On the other hand, according to the one-component system developing method, in the magnedry method using magnetic toner and the contact developing method using no magnetic toner, the toner is contacted with the whole surface to be developed, namely irrespectively of the image portion and the non-image portion. For this reason, toner is readily attached even to the non-image portion, thereby causing contamination called ground fog. (This problem of fog contamination was also similarly observed in the two-component system developing method.) Also, the powder cloud method cannot be free from attachment of the toner particles in powdery state onto the non-image portion, thus involving also the drawback that the ground fog cannot be removed.
In this respect, the developing method as proposed in Japanese Laid-open Patent Application No. 43027/1979 and No. 18656/1980, which comprises disposing an electrostatic image bearing member which bears an electrostatic image on its surface and a developer carrying member which carries an insulating developer on its surface at a developing section with a certain gap provided therebetween, carrying a developer to a thickness thinner than the above gap on the developer carrying member and transferring the developer onto the electrostatic image bearing member, has an advantage that the fog as described above will hardly appear. This method, because no carrier particle is used, causes no change in mixing ratio as mentioned above and further no deterioration of carrier particle and therefore may be stated to be an electrostatic image developing method which can give highly faithful images of stable quality.
In the jumping developing method known in the art, when copying is continued repeatedly, evenness of the developer layer carried on a developer carrier may sometimes be impaired, whereby various inconveniences may be caused. For example, coating failure in streaks in the circumferential direction of the carrier may be generated, or the thickness of the developer layer may become extremely thinner partially as compared with the initial thickness to result in generation of irregularities like speckles or generation of coating failure like ripples. The former is observed as white streaks on the image on development, while the latter as density irregularities like speckles or like ripples. This phenomenon will scarcely occur in ordinary repeated copying, but it may sometimes occur undesirably in continual use, particularly under environmental conditions of extremely low temperature and low humidity for a long term.
Also, under higher temperature and higher humidity conditions, the thickness of the developer layer may sometimes unfavorably change and become thinner in most cases, thus frequently causing lowering of image density. As the result of investigations about this point, it has been found that one cause thereof resides in insufficient stability and reliability of the charge controller component, and that characteristics of adhesion of the developing powder onto the sleeve and transfer of the developing powder from the sleeve may be changed by such causes.
Explaining in further detail, such phenomena are caused by generation of portions having uneven triboelectric charges in the developer layer carried on the carrying member due to the change in the environmental conditions. More specifically, under the environmental conditions of extremely low temperature and humidity, a component of the developer with extremely large triboelectric charges is formed through friction of the carrying member surface and the developer. Due to the image force caused by the charges, such a component with extremely large triboelectric charges is liable to be accumulated in the vicinity of the carrying member and affect influences on evenness or readiness in developing of the upper layer portion of the developer, thereby causing such irregularities as white streaks, speckle-like irregularities, and ripple patterns as mentioned above. The reduction in thickness of the developer layer at higher temperature and humidity may also be generated by uneven triboelectrification between the developer and the carrying member, namely due to instability of the amount of triboelectric charges of the developer near the surface of the carrying member.
Positive charge controllers used in toners for dry system development are generally, for example, quaternary ammonium compounds and organic dyes, particularly basic dyes and salts thereof. Positive charge controllers conventionally used are benzyldimethylhexadecylammonium chloride, decyl-trimethylammonium chloride, nigrosine base, nigrosine hydrochloride, safranine .gamma., crystal violet and others. Particularly, nigrosine base and nigrosine have been frequently used as positive charge controllers. These are usually added to a thermoplastic resin to be dispersed in the resin while it is molten under heating, and the resultant resin mixture is micropulverized into fine particles, adjusted to suitable sizes, if desired, and then provided for use.
However, these dyes as charge controllers have complicated structures and do not have constant properties, thus being poor in stability. Also, decomposition or denaturation may occur through decomposition, mechanical collision and friction during kneading under heat or change in temperature and humidity conditions, to cause lowering in the charge controlling characteristic.
Accordingly, when development is carried out by use of a toner containing these dyes as charge controllers in a copying machine, the dyes may undergo decomposition or denaturation as the increase in number of copies to cause deterioration of the toner during continual use.
Also, most of the substances known to be positively chargeable are generally dark in color and have a disadvantage that they cannot be incorporated in a brilliant chromatic developer.
As another vital disadvantage, it is very difficult to disperse these dyes as charge controllers evenly into a thermoplastic resin, resulting in different amounts of triboelectric charges among the toner particles obtained by pulverization. For this reason, in the prior art, various methods have been practiced in order to disperse these dyes more evenly into a resin. For example, a basic nigrosine dye is formed into a salt with a higher fatty acid for improvement in compatibility with a thermoplastic resin. In this case, however, unreacted fatty acid or dispersed product of the salt will be exposed on the toner surfaces to contaminate carriers or toner carrying member and also cause lowering in free flowing property of the toner, fog and lowering in image density. Alternatively, for improvement in dispersibility of these dyes into a resin, there is also employed a method in which dye powders and resin powders are previously mechanically pulverized and mixed before fusion kneading. This method is not competent enough to overcome the original poor dispersibility, and evenness of charging satisfactory in practical application had not yet been obtained.
Furthermore, most of dyes for positive charge controlling are hydrophilic and therefore, due to poor dispersibility of these dyes into a resin, the dyes are exposed on the toner surfaces when pulverized after fusion kneading. Accordingly, when the toner is used under highly humid conditions, a drawback that no image of good quality can be obtained is involved because of such hydrophilic nature of the charge controller.
Thus, when a dye having positive charge controller of the prior art is used in a toner, variances in amount of the charges generated on the toner particles surfaces will occur among toner particles, between toners and carriers or between toners and toner carrying member such as sleeve, whereby various inconveniences are caused, such as fogging, toner scattering, carrier contamination, etc. These difficulties become marked when a large number of copying cycles were carried out continuously, giving the results essentially unsuitable for a copying machine.
Also, most of the toners are unsuitable for use under highly humid conditions, because the transfer efficiency of the toner image is markedly lowered. Even under normal temperature and humidity, when the toner is stored for a long time, due to instability of the positive charge controller employed, most of toners may be denatured to become useless.